<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://cdn.bootcdn.net/ajax/libs/vue/2.6.9/vue.min.js"></script>
</head>
<body>
    <div id="app">
        <demo></demo>
    </div>
    <template id="demo">
        <h1>hhhh</h1>
    </template>
    <script>
        Vue.component("demo",{
            template:"#demo",
        })
        let vm=new Vue({
            el:"#app",
        })
    </script>
</body>

</html>